The RPi, even the 3, isn't a great choice for networked storage, since the NIC is running over the USB bus. A Beaglebone Black[1] would be similar but have better hardware for the job.
I have a Beaglebone Black with a WD USB 2 portable HDD attached to it. I get only around 5 MB/s while copying (from Beagle to PC) a ~500 MB file over 5 GHz wifi. Not sure if this is because of USB 2.0 though.
My understanding is that the Beaglebone Black is also... opener. Or at least it seems like the Beaglebone Black is supported by more operating systems. If, for example, you wanted to run OpenBSD or even Minix 3, BBB is the system for you.
A chromebox with a new seabios rom (so it can run a normal Linux distro) would be a decent compromise between those two extremes.
I have an Asus chromebox doing this, very happy with it.
USB drives and ethernet share the same bus on RPi so NAS performance is not great.